Downhole apparatus

ABSTRACT

Downhole apparatus in the form of a valve ( 140 ) comprises a body ( 148 ) defining a bore ( 168 ) with a curved disc closure member ( 142 ) positioned in the bore. First and second retaining members ( 144, 146 ) are positioned on respective sides of the disc ( 142 ) for retaining the disc in a closed position and to hold pressure from both sides. One of the retaining members ( 144 ) is retractable to permit opening of the valve disc ( 142 ). The disc ( 142 ) may be locked closed by locking the retractable retaining member ( 144 ) in position. On release of the member ( 144 ), application of fluid pressure to portions of the member ( 144 ) will retract the member ( 144 ).

This invention relates to apparatus for use in downhole operations. Inparticular, but not exclusively, the apparatus relates to an isolationvalve intended for use in completion testing and in operations whichtake place immediately following completion testing.

In the oil and gas exploration and extraction industries, deep bores aredrilled to gain access to hydrocarbon-bearing strata. The section ofbore which intersects this strata or “production zone” is typicallyprovided with a steel “liner”, while the section of bore extending tothe surface is lined with steel “casing”. Oil and gas is extracted fromthe production zone through production tubing extending through thecasing from the upper end of the liner. The production tubing is formedof a string of threaded sections or “subs” which are fed downwards fromthe surface, additional subs being added at the surface until the stringis of the desired length. As the string is assembled and fed into thebore its pressure integrity, or “completion”, is tested at regularintervals. Such testing is also carried out on the complete string. Thetesting is accomplished by pressurising the internal bore of the string.Of course this requires that the string bore is sealed at its lower end.

This sealing of the string bore is generally accomplished using a valveor plug which will normally remain closed or in place once testing iscompleted, to allow the packers mounted on the string to be set tolocate and seal the string within the casing or liner. The valve or plugmay then be opened or removed to permit formation fluid to flow upwardlyto the surface through the production tubing. The opening or removaloperation generally requires running in of an appropriate tool on, forexample, wireline or coiled tubing, which will involve additional timeand expense.

It is among the objectives of embodiments of this invention to obviateor mitigate these disadvantages. It is a further objective ofembodiments of this invention to provide an isolation valve which willhold pressure in two directions, that is from the sump side and thesurface side.

According to a first aspect of the present invention there is provided adownhole valve comprising a body defining a bore, a valve closure memberpositioned in the bore, first and second retaining members positioned onrespective sides of the valve closure member for retaining the valveclosure member in a closed position and to hold pressure from bothsides, one of the retaining members being retractable to permit openingof the valve closure member.

In use, such a downhole valve will hold pressure from both the surfaceside and the sump side. The terms “above” and “below” are used herein,but those of skill in the art will of course realise that the inventionmay be used with equal utility in inclined or horizontal bores, and theorientation of the valve may be varied.

Preferably, with the valve closure member in the open position, the bodydefines a slick bore.

Preferably also, the valve closure member comprises a flapper in theform of a disc. Most preferably, the disc is in the form of a curved orconcave disc. In the preferred embodiment a convex surface of the discengages a valve seat.

Preferably also, one of the retaining members is extendable to maintainthe valve closure member in an open position and to provide a slickbore. In the preferred embodiment one of the retaining members is bothretractable, to permit opening of the valve closure member, andextendable to maintain the valve closure member in the open position.

Preferably also, one of the retaining members incorporates a valve seat.The valve seat may include an elastomeric seal located in an end surfaceof the retaining member. Most preferably, the retaining memberincorporating the valve seat is non-retractable. Alternatively, aseparate valve seat may be provided.

Preferably also, the retractable retaining member is moveable byapplication of fluid pressure thereto. The fluid pressure may beprovided by well fluid in the borehole, and most preferably by the wellfluid in the body bore. The supply of fluid from the body bore toactuate the retaining member may be controlled by an appropriate valve,such as described in PCT\GB95\02046.

Preferably also, the retractable retaining member includes a sleeveportion defining a piston, such that application of fluid pressurebetween the sleeve portion and the body tends to retract the member froma retaining position. The member may be biassed towards the retainingposition by biassing means, such as a spring.

Preferably also, the retractable retaining member is lockable in theretaining position, but is releasable, most preferably on application ofactuating fluid pressure. Most preferably, the unlocking of theretractable retaining member is controlled by a ratchet assemblycomprising first and second axially relatively movable parts, each partdefining a toothed face, and a ratch member located between the toothedfaces, pressure induced reciprocal movement of the parts advancing theratch member axially along the toothed face of the first part, in anadvanced position the ratch member engaging a unlocking member such thatfurther movement of the first part actuates the unlocking member torelease the retaining member.

Additionally, or alternatively, the retractable retaining member may bereleasable by application of physical force by a further tool located inthe bore. Preferably, the unlocking member defines a tool engagingprofile for cooperating with said further tool.

Preferably also, the retractable retaining member may be latched in theretracted position, to permit opening of the valve closure member, andthen released to return to an extended position to maintain the valveclosure member open.

Preferably also, the valve closure member is in the form of a flapperand is mounted on a valve carriage which, with the retaining memberretracted, is axially movable towards the retaining member such that theretaining member may contact the flapper and push the flapper towardsthe open position. The valve carriage and the retaining member arepreferably connected by a resilient link. In the preferred embodiment,retraction of the retaining member is achieved by pressurising the bore,which also maintains the valve carriage and flapper in the closedposition, with the flapper in sealing contact with the other retainingmember. Bleeding off bore pressure following retraction of the retainingmember allows the flapper to lift from the other retaining member andthe valve carriage to follow the retracted retaining member, and the endof the retaining member to contact the flapper and push the flapper tothe open position.

The valve may include vent means for equalising pressure across thevalve closure member prior to the retractable retaining memberpermitting opening. The vent means may be openable by initialapplication of fluid pressure, to permit fluid communication across thevalve member. Most preferably, the vent means includes a moveablemember, such as a sliding sleeve, which initially closes a vent passagebut is moveable to open the passage. Preferably also, the moveablemember also serves, in its initial position, to lock the retractableretaining member in the retaining position.

The other of the retaining members may be biassed to move the valveclosure member to the open position. Alternatively, the valve closuremember may be provided with means for biassing the member towards theopen position.

According to another aspect of the present invention there is provided amethod of completing a downhole string including the steps of:

providing a valve in a tubular string, which valve is capable of holdingpressure from both above and below;

running the string into a bore with the valve closed;

securing the string in the bore; and

opening the valve to permit flow of fluid through the string.

Conventionally, in a completion operation, the string is provided with anormally-closed valve which opens in response to higher pressure in thewell to permit well fluid to flow into the string. Thus, such valves arenot suitable for use as safety valves, and separate safety valves mustbe provided in the string to safeguard against surges of fluid upthrough the string when upper end of the string is opened. A valve whichwill hold pressure from both the sump and surface sides allows forcompletion testing against the valve and may also serve as a safetyvalve. In the method of this aspect of the invention top filling may beutilised for filling the string with fluid as it is run into the bore.

According to a further aspect of the present invention there is provideda downhole valve comprising: a curved valve closure member defining aconvex face and a seal area on said face; and a valve seat for engagingthe seal area.

Preferably, the valve includes a tubular valve body having a main axisand the seal area defines a surface which is substantially perpendicularto said axis.

According to a still further aspect of the present invention there isprovided a downhole valve for holding fluid pressure in a firstdirection, the valve including a non-planar valve closure memberdefining a peripheral seal surface and a valve seat having acorresponding sealing area, the seal surface and sealing area beingsubstantially perpendicular to said first direction.

It has been found that the sealing capabilities of valves in accordancewith this aspect of the invention compare favourably in comparison withvalves in which the seal surface simply coincides with the surface ofthe valve closure member.

Preferably, the valve closure member is a curved flapper, and mostpreferably the seal surface is provided at the periphery of the convexface of the member.

Preferably also, the sealing area includes a resilient sealing portion.

According to a yet further aspect of the present invention there isprovided a ratchet arrangement for downhole apparatus, the arrangementcomprising first and second parts, each part defining a toothed face,and a ratch member located between the toothed faces, the parts beingaxially relatively movable by application of fluid pressure thereto,wherein reciprocal movement of the parts advances the ratch memberaxially along the toothed faces.

The ratch member may engage a part of another tool or device on reachingan advanced position, and serve to actuate the tool or device ortransfer force thereto from one of the parts. In a preferred embodimentthe ratch member is utilised to transfer force from the first part tounlock a further part of a valve to permit opening of the valve.

Preferably, the first part defines a piston and is movable onapplication of fluid pressure thereto, and the first part has a strokecorresponding to the tooth spacing on the toothed faces of the partssuch that each pressure cycle will advance the ratch member one tooth.Accordingly, by providing a known number of teeth and knowing theinitial position of the ratch member, the ratch member may be moved to apredetermined advanced position by application of a predetermined numberof pressure cycles. This feature is useful when used in conjunction withpressure actuated tools for use in completion operations, where pressureis used in, for example, completion testing and setting packers. Usingthis aspect of the invention, the operation of a particular pressureactuated tool, such as an isolation valve, may be controlled by theratchet assembly, and will only commence after a predetermined number ofpressure cycles, thus accommodating completion testing operations andthe setting of packers.

The piston and toothed face of the first part may be integral.Alternatively, the toothed face may form part of a unit, incorporatingthe other toothed face and the ratch member, which is separable from thetool or device provided in combination with the ratchet assembly.

Preferably, the ratch member comprises first and second portions and aspring portion acting therebetween to urge the first and second portionsinto engagement with the respective toothed faces. Preferably, the ratchmember portions are combined as a single integral part.

These and other aspects of the present invention will now be described,by way of example, with reference to the accompanying drawings, inwhich:

FIG. 1 is a sectional view of a downhole isolation valve in accordancewith a first embodiment of the present invention;

FIG. 1A illustrates the true cross-section at area 1A of FIG. 1;

FIG. 2 is a sectional view on line 2—2 of FIG. 1;

FIG. 3 is an enlarged view of a portion of the isolation valve of FIG.1, with the valve closure member in the closed position;

FIGS. 4, 5 and 6 are sectional views corresponding to FIG. 3, andillustrating the sequence of events culminating in the valve beinglocked open; and

FIG. 7 is a sectional view of a downhole isolation valve in accordancewith a second embodiment of the present invention;

FIGS. 8 and 9 are sectional views of a downhole isolation valve inaccordance with a preferred embodiment of the present invention;

FIG. 10 is a view from below of the valve disc of the valve of FIG. 8;

FIG. 11 is a side view of the disc of FIG. 10;

FIG. 12 is a sectional view on line 12—12 of FIG. 10;

FIG. 13 is a plan view of the lower retaining sleeve of the valve ofFIG. 8;

FIG. 14 is a sectional view on line 14—14 of FIG. 13;

FIG. 15 is an enlarged sectional view of a ratch member of the valve ofFIG. 8 (on the same sheet as FIG. 8);

FIGS. 16 and 17 are sectional views of a portion of a valve forincorporating a ratch assembly in accordance with a preferred embodimentof another aspect of the invention;

FIG. 18 is a side view of a ratch assembly for incorporation with thevalve of FIG. 16;

FIG. 19 is an end view of a toothed track of the assembly of FIG. 18;and

FIG. 20 is a plan view of the ratch assembly of FIG. 18.

Reference is first made to FIGS. 1 to 6 of the accompanying drawings,which illustrate a downhole isolation valve 20 in accordance with afirst embodiment of the present invention.

The isolation valve 20 comprises a tubular body 22 provided with upperand lower end caps 24, 25 provided with threaded ends for locating thebody 22 in a tubular string (not shown). A valve member in the form of aconcave circular disc 26 is mounted towards the upper end of the body22, and is initially locked closed. The disc 26 seals against the endsof corresponding profiled upper and lower sliding sleeves 30, 31defining respective seals 32, 33. The seals 32, 33 are pre-loaded by acompression spring 34 located on the lower or sump side of the disc 26,and acting between the lower end of the sleeve 31 and the lower end cap25, to provide low pressure sealing. Pressure on the upper or surfaceside loads a snap ring 36 which locks the lower end of the sleeve 31relative to the body 26 (see FIG. 1A for true cross-section at snap ring36). Pressure from the sump side loads the upper end cap 24, via theupper sleeve 30. A compression spring 37 is provided between the upperend cap 24 and the upper sliding sleeve 30 and is used in opening thedisc 26, as will be described.

There is an annular volume 38 defined between the inner wall of the body22 and the outer wall of the lower sleeve 31. The volume 38 accommodatestwo sleeves: a disc mounting sleeve 40, to which the disc 26 is hingedand which is fixed to the body 22; and a sliding vent sleeve 42 which isaxially movable within the volume 38. Rotational movement of the sleeve42 is restricted by a guide pin 44 extending through the sleeve 31. Inits initial position the vent sleeve 42 closes a vent passage 46 linkingthe volume 38 with a volume 48 on the surface side of the disc 26 whichaccommodates the spring 37. The sleeve 42 is initially fixed at thelower end of the volume 38 and is held in position by a shear pin 50.The sleeve 42 defines an annular groove 52 on its outer face whichaccommodates the snap ring 36 in its locked position. The sleeve 42defines a shoulder 56 positioned above the outlet of a fluid passage 58which communicates, through appropriate control lines and valves, to asupply of pressurised fluid or, most preferably, to a respective shuttlevalve on a control tool as described in PCT\GB95\02046 orPCT\GB96\01907, the disclosures of which are incorporated herein byreference; the shuttle valve permits fluid communication between thebody bore and the passage 58.

As noted above, the valve 20 is run in the closed position with the sumpside compression spring 34 providing a low pressure sealing force.Pressure from the sump side acts over seals 32, 33 and also a seal 60between the lower end of the sleeve 31 and the body 22. The loadgenerated by this pressure is supported by the upper end cap 24.Pressure from the surface side acts over the seals 32, 33 and also theseal 62 between the upper end of the sleeve 30 and the body 22.

To open the valve 20, a control tool (not shown) as described above issubject to a predetermined number of pressure cycles to open theappropriate shuttle valve, allowing pressurised well fluid to flow intothe passage 58. This pressure acts on the lower sliding vent sleeveshoulder 56, shears the pin 50 and moves the sleeve 42 upwardly in thevolume 38 lifting the upper end of the sleeve 42 clear of the ventpassage 46, and permitting fluid communication over the disc 26 andallowing the pressure to balance between each side of the disc 26.Upward movement of the sliding vent sleeve 42 also unlocks the snap ring36.

With the snap ring 36 unlocked, the lower sliding sleeve 31 can nowretract as the hydraulic fluid pressure force created in the volume 38overcomes the biassing force produced by the spring 34. When the lowersleeve 31 is fully retracted, the upper sliding sleeve 30 forces thedisc 26 open under spring force.

On hydraulic pressure being bled off from the volume 38, the lowersliding sleeve 31 is returned to its initial position by spring force.As the lower sliding sleeve 31 returns to its initial position itretains the disc 26 in the open position, and provides a slick bore.

Reference is now made to FIG. 7 of the drawings, which illustrates adownhole isolation valve in accordance with a second embodiment of thepresent invention. The valve 70 comprises a tubular body 72 comprisingan outer sleeve 73 with upper and lower end caps or sleeves 74, 75threaded to the ends thereof. A valve member in the form of a concavecircular disc 76 is mounted towards the lower end of the body 72, and isinitially locked closed, as illustrated in FIG. 7. In the closedposition the convex disc surface 77 is in sealing contact with a valveseat 78 defined by the upper end of a lower retaining sleeve 80. Theseat 78 includes a groove which accommodates an elastomeric seal 79. Thelower end sleeve 75 provides a mounting for the retaining sleeve 80 anda sealing O-ring 82 is provided therebetween.

The disc 76 is retained in the closed position, against the valve seat78, by an upper retaining sleeve 84 having a lower end which correspondsto the concave face 86 of the disc 76. Initially, with the disc 76locked closed, the upper end of the retaining sleeve 84 is fixed againstaxial movement relative to the outer sleeve 73 by a split\snap ring 88located in an external annular groove 90 in the sleeve 84 and engagingan internal groove 92 on the inner wall of the outer sleeve 73. Part ofthe retaining sleeve groove 90 is formed in the upper portion of anactuator sleeve 94, the lower portion of which is slightly enlarged andforms a piston within an annular chamber 96 between the outer wall ofthe retaining sleeve 84 and the inner wall of the outer sleeve 73. Thespace between the retaining sleeve 84 and the outer sleeve 73 above theactuator sleeve 94 is in communication with a pressurising fluid linefor connection to a control line (not shown) linked to a pressurisedfluid source. The control line leads into a fluid communication line 98formed through the upper end sleeve 74 and which line 98 continuesthrough the upper end of the outer sleeve 73 and opens into a smallchamber 100 at the upper end of the retaining sleeve 84. Thus,application of fluid pressure through the line 98 into the chamber 100will force the actuator sleeve 94 downwardly and push the split ring 88radially outwardly and fully into the groove 92, thus unlocking theretaining sleeve 84 from the outer sleeve 73.

On release of the split ring 88, the retaining sleeve 84 will not beimmediately retracted, as the sleeve 84 is biassed into the retainingposition by a compression spring 104 provided in a spring housing 106and which acts between the lower face of the upper end sleeve 74 and ashoulder 108 on the housing 106. However, by increasing the pressurethat is applied through the line 98 into the chamber 100 an upwardlydirected pressure force will act against the lower side of the springhousing shoulder 108 and above a predetermined bore\annulus pressuredifferential this pressure force will overcome the retaining springforce and retract the retaining sleeve 84. The retraction of the sleeve84 continues until a set of latch fingers 110 engage an annular groove112 on the outer surface of the retaining sleeve 84. The latch fingers110 are mounted on a sleeve 114 located in the chamber 96 and which isfixed relative to the outer sleeve 73 by anchor pins 116.

The disc 76 is mounted, via a hinge pin 117, to a valve sleeve orcarriage 118 which is axially movable within the chamber. The carriage118 is threaded to the lower end of a trigger sleeve 120 linked to theretaining sleeve 84 via a compression spring 122; the spring 122 actsbetween a shoulder 124 towards the upper end of the sleeve 120 and acollar 126 fixed to the retaining sleeve 84. A retaining sleeve 128extends upwardly from the upper end of the trigger sleeve 120.

When the retaining sleeve 84 is retracted as described above, byapplication of bore pressure through the line 98 to the chamber 100, andhas been latched in the retracted position by the latch fingers 110, thepressure within the bore retains the disc 76 in the closed position andin contact with the valve seat 78. However, the relative axial movementbetween the retaining sleeve 84 and the valve carriage 118 on retractionof the sleeve 84 results in compression of the spring 122. Accordingly,as pressure is bled off from the bore, and the pressure differentialacross the disc falls, the disc 76 will be lifted from the valve seat 78by the extension of the spring 122. The upward movement of the valvecarriage 118 and disc 76 continues until the upper concave disc face 86contacts the lower end of the retaining sleeve 84, which contact causesthe disc 76 to be pivoted to the open position.

Once the disc 76 has been pushed to the fully open position, the upperend of the trigger sleeve 120 comes into contact with the latch fingers110 and lifts the fingers 110 out of the groove 112 to latch with thefinger retaining sleeve 128, such that the valve disc retaining sleeve84 is free to move downwardly once more under the influence of thespring 104. The freed retaining sleeve 84 moves downwardly, to isolatethe disc 76 between the sleeve and the outer sleeve 73, and also suchthat the lower end of the sleeve 84 comes into contact with the valveseat 78. The valve is now held in the open position, with the sleeve 84defining a slick bore past the open disc 76.

In the event that, for whatever reason, it is not possible to open thevalve solely by application of fluid pressure, a mechanical overridesleeve 130 is provided within the valve bore at the upper end of theretaining sleeve 84. The outer wall of the sleeve 130 defines a groove132. A number of balls 134 are provided in the groove and extend throughcorresponding openings 136 in the retaining sleeve 84 and contact theinner surface of the split ring 88. Accordingly, when the sleeve 130 ispulled upwardly using a suitable downhole tool, the balls 134 are pushedoutwardly through the openings 136 to push the split ring 88 into theouter sleeve groove and release the retaining sleeve 84 from the outersleeve 73. Further upward movement of the sleeve 130 will lift theretaining sleeve 84 and permit the disc 76 to open, as described above.

It will be evident that the valve 70 described above will hold pressurefrom both the surface and sump sides, but may be opened when desiredeither by application of bore pressure or by mechanical means, toprovide an unrestricted or slick bore.

Reference is now made to FIGS. 8 and 9 of the drawings, which illustratean isolation valve 140 in accordance with a preferred embodiment of thepresent invention. Like the embodiments described above, the valve 140features a concave valve disc 142 and upper and lower retaining membersor sleeves 144, 146, and the disc mounting and retaining arrangement issubstantially similar to the valve 70. However, this valve 140 isoperated in a somewhat different manner, in that the retractableretaining sleeve 144 is actuated by the pressure of well fluid directlyabove the disc 142 without requiring the provision of a separate controltool, and the control of the unlocking of the sleeve 144 is accomplishedby an arrangement forming part of the valve 140, rather a separatecontrol tool, as will be described. FIG. 8 illustrates the valve 140 ina well bore 141, positioned below a packer 143.

If reference is made also to FIGS. 10 to 14 of the drawings, the disc142 and lower retaining member 146 are illustrated in greater detail.The concave disc 142 defines a peripheral sealing area 147 on its convexface which surface is perpendicular to the disc Z-axis. The sleeve 146defines a corresponding valve seat 149, defining a groove to receive aresilient seal member.

As with the valve 70 described above, the retractable sleeve 144 islocked in position relative to the valve body 148 by a lock ring 150. Alock sleeve 152 holds the lock ring 150 in a radially extendedconfiguration in a groove 154 in the valve body 148 and in this positiona shoulder 156 defined by the sleeve 144 abuts the ring 150, restrictingupward axial movement of the sleeve 144. The lock sleeve 152 may belifted to release the lock ring 150, and thus release the retainingsleeve 144, by a ratch member 158 which is movable along an axial slot160 in the lock sleeve 152. The ratch member 158 is located between twoopposing toothed surfaces 162, 163 provided on respective sleeves 164,165. The inner sleeve 164 is threaded to the upper end of theretractable retaining sleeve 144, while the outer sleeve 165 is movableindependently of the sleeve 144, but is biassed downwardly by acompression spring 166 which acts between the upper end of the sleeve165 and a shoulder formed on the valve body 148. The interaction of theratch member 158 with the toothed surfaces 162, 163, allows a number ofpressure cycles to be applied to the valve 140 before the retainingsleeve 144 is unlocked to allow opening of the disc 142, as describedbelow.

When the string and thus the valve bore 168 is pressurised, fluidpressure acts on two piston areas 170, 171. The first piston area 170 isformed on the retractable retaining sleeve 144 and is in communicationwith the valve bore 168 via ports 172. However, while the sleeve 144 islocked relative to the valve body 148 by the lock ring 150, no movementof the sleeve 144 may take place. The second piston area 171 is definedby the sleeve 165 and is in communication with the valve bore 168 viaports 173 in the inner sleeve 164. Application of a fluid pressure forceto the piston area 171 lifts the sleeve 165, against the action of thespring 166, until a split collar 174 located in an annular groove in theupper end of the sleeve 165 engages a shoulder 176 defined by the innersleeve 164. This “stroke” of the sleeve 165 corresponds to the length ofone of the teeth of the toothed surface 163. Thus, as the sleeve 165 islifted by application of well fluid pressure, the ratch member 158 isalso lifted a corresponding distance, however when the fluid pressure inthe valve bore 168 is reduced, and the spring 166 moves the sleeve 165downwards, the ratch member 158 is retained in its advances position bythe toothed surface 162 of the inner sleeve 164.

If reference is made to FIG. 15 of the drawings, it will be noted thatthe ratch member 158 comprises two inter-fitting part annular segments180, 181 which are urged into a radially extended position by a coilspring 182. A guide pin 184 is fixed to the inner segment 180, andextends through an opening in an outer segment 181. The pin 184corresponds with an axial slot 186 in the outer sleeve 165.

With each pressure cycle that is applied to the string, the ratch member158 is advanced one step along the inner sleeve toothed surface 162.After a predetermined number of cycles, the ratch member 158 reaches theend of the lock sleeve groove 160, such that the next increase inpressure within the string and valve bore 168 will result in the ratchmember 158 lifting the lock sleeve 152, allowing the lock ring 150 tocontract radially, and thus freeing the retaining sleeve 144 from thevalve body 148. The subsequent sequence of events is similar to thatdescribed with reference to the valve 70 described above, as describedbriefly below.

Once the retaining sleeve 144 has been released from the body 148, thefluid pressure acting on the piston area 170 will tend to lift thesleeve 144 relative to the valve body 148, bringing the inner sleeve 164into contact with the outer sleeve 165 at the piston area 171, such thatsubsequent movement of the sleeve 144 is resisted by the action of thespring 166. While the sleeve 144 moves upwardly, the disc 142 ismaintained in contact with the valve seat 149 defined by the lowermember 146 by the pressure acting downwardly on the disc 142. As withthe above-described embodiment, the disc 142 is mounted on a carriage188 linked to the sleeve 144 via a trigger sleeve 190 and a spring 192.The retraction of the sleeve 144 continues until latch fingers 194mounted on the valve body 148 engage a profile 196 on the sleeve 144.

If pressure is then bled off from the valve bore 168 above the disc 142,the pressure force maintaining the disc 142 in contact with the lowerseat falls, until, when the pressure across the disc 142 is almostequalised, the spring 192 lifts the carriage 188 and disc 142 towardsthe end of the sleeve 144. The upper surface of the disc 142 will thenbe brought into contact with the lower end of the sleeve 144 and will bepushed into the open position. When the disc 142 is fully open, atrigger nose 197 provided on the upper end of the trigger sleeve 190releases the latch fingers 194, such that the action of the spring 166pushes the sleeve 144 downwardly to retain and isolate the disc 142 inthe open position.

Reference is now made to FIGS. 16 to 20 of the drawings, whichillustrate part of a valve 198 and a modified ratch assembly 200, inaccordance with aspects of the present invention. The ratch assembly 200operates in a manner which is substantially the same as the ratchassembly described above, however, this assembly 200 includes a unit 202(FIG. 18), consisting of the first and second toothed tracks 204, 205and the ratch member 206, which is removable from the remainder of thedevice. The sleeve 208 incorporating the piston 210 which inducesmovement of the first toothed track 204 is mounted on the valve,separately from the unit 202, and may be connected to an upper portionof the track 204 using an appropriate fastener.

The unit 202 is located in the valve by passing the unit 202 through asuitable door in the valve body (not shown) into a longitudinallyextending aperture 212 in an upper portion of the unlocking member 214(FIG. 17 illustrates the position of the unlocking member 214 after ithas been lifted by the ratch member 206).

The ratch member 206 is formed of a single wedge-shaped block of metalin which a key-hole slot has been cut to permit deformation of the blockas it climbs the tracks 204, 205.

In use, two units 202 will be fitted to the valve after the assembledvalve has been tested, such that there is no requirement to reset theratch members following testing. This provides an additional advantagein that it is no longer necessary to form a slot in the valve body alongthe length of the toothed tracks, as required in the above describedembodiment, to allowing resetting of the ratch member; the presence ofthe slot leads to a weakening of the valve body.

It will be clear to those of skill in the art that the valves describedabove may be used in many downhole applications, and offer manyadvantages over conventional isolation valves and plugs. The valves maybe opened merely by appropriate application of bore pressure, and thusobviate the need for intervention using, for example, wireline mountedtools. Further, the valves may be located at any convenient location ina string and may be positioned below a packer or other apparatus ifdesired. It will further be clear to those of skill in the art that theabove-described embodiments are merely exemplary of the presentinvention, and that various modifications and improvements may be madethereto without departing from the scope of invention as defined in theappended claims.

What is claimed is:
 1. A downhole valve comprising: a body defining abore; a valve closure member positioned in the bore; first and secondretaining members positioned on respective sides of the valve closuremember for retaining the valve closure member in a closed position andto hold pressure from both sides, one of the retaining members beingboth retractable to an open position to permit opening of the valveclosure member and extendable from the retracted position to maintainthe valve closure member in the open position.
 2. The valve of claim 1wherein, with the valve closure member in the open position, the bodydefines a slick bore.
 3. The valve of claim 1 wherein the valve closuremember is a flapper.
 4. The valve of claim 3 wherein the flapper is inthe form of a concave disc.
 5. The valve of claim 4 wherein a convexsurface of the disc engages a valve seat.
 6. The valve of claim 1wherein one of the retaining members is movable to maintain the valveclosure member in said open position.
 7. The valve of claim 1 whereinone of the retaining members incorporates a valve seat.
 8. The valve ofclaim 1 wherein the retractable retaining member is movable byapplication of fluid pressure force thereto.
 9. The valve of claim 8wherein the retractable retaining member is retractable by applicationof bore pressure.
 10. The valve of claim 1 wherein the retractableretaining member includes a sleeve portion defining a piston, such thatthe application of fluid pressure between the sleeve portion and thebody tends to retract the member from the closed position.
 11. The valveof claim 10 wherein means is provided for biassing the retractableretaining member towards the closed position.
 12. The valve of claim 1wherein the retractable retaining member is lockable in the closedposition, but is releasable on application of actuating fluid pressure.13. The valve of claim 1 wherein the retractable retaining member islockable in the closed position, but is releasable by application ofphysical force thereto by a further tool located in the bore.
 14. Adownhole valve comprising: a body defining a bore; a valve closuremember positioned in the bore; first and second retaining memberspositioned on respective sides of the valve closure member for retainingthe valve closure member in a closed position and to hold pressure fromboth sides, one of the retaining members being retractable to permitopening of the valve closure member, the retractable retaining memberbeing movable by application of body bore fluid pressure force theretoand wherein the supply of fluid from the body bore to actuate theretaining member is controlled by a remotely actuatable valve.
 15. Adownhole valve comprising: a body defining a bore; a valve closuremember positioned in the bore; first and second retaining memberspositioned on respective sides of the valve closure member for retainingthe valve closure member in a closed position and to hold pressure fromboth sides, one of the retaining members being retractable to permitopening of the valve closure member, and wherein release of theretractable retaining member is controlled by a ratchet assemblycomprising first and second axially relatively movable parts, each partdefining a toothed face, and a ratch member located between the toothedfaces, pressure induced reciprocal movement of the parts advancing theratch member axially along the toothed face of the first part, in anadvanced position the ratch member engaging a unlocking member such thatfurther movement of the first part actuates the unlocking member torelease the retractable retaining member.
 16. A downhole valvecomprising: a body defining a bore; a valve closure member positioned inthe bore; first and second retaining members positioned on respectivesides of the valve closure member for retaining the valve closure memberin a closed position and to hold pressure from both sides, one of theretaining members being retractable to permit opening of the valveclosure member, and wherein the retractable retaining member islatchable in a retracted position, and releasable from the retractedposition to allow movement of the retractable retaining member to aposition to maintain the valve closure member in the open position. 17.A downhole valve comprising: a body defining a bore; a valve closuremember positioned in the bore; first and second retaining memberspositioned on respective sides of the valve closure member for retainingthe valve closure member in a closed position and to hold pressure fromboth sides, one of the retaining members being retractable to permitopening of the valve closure member, and wherein the valve closuremember is in the form of a flapper and is mounted on a valve carriagewhich, with the retractable retaining member retracted, is axiallymovable towards the retractable retaining member such that theretractable member contacts the flapper and pushes the flapper towardsthe open position.
 18. The valve of claim 17 wherein the valve carriageand the retractable retaining member are connected by a resilient link.19. The valve of claim 17 wherein the retractable retaining member isarranged to be retractable by application of bore pressure, which borepressure also maintains the valve carriage and flapper in the closedposition, and whereby bleeding off the bore pressure allows the valvecarriage to follow the retractable retaining member and further allowingan end of the retractable retaining member to contact the flapper andpush the flapper to the open position.
 20. A ratchet arrangement fordownhole apparatus, the arrangement comprising first and second parts,each part defining a toothed face, and a ratch member located betweenthe toothed faces, the parts being axially relatively movable, whereinreciprocal movement of the parts advances the ratch member axially alongthe toothed faces and wherein the parts are axially relatively movableby application of fluid pressure thereto.
 21. The arrangement of claim20, wherein the ratch member engages a part of an associated tool onreaching an advanced position, to actuate the tool by transferring forcethereto from one of the parts.
 22. The arrangement of claim 20, whereinthe first part defines a piston and is movable on application of fluidpressure thereto, and the first part has a stroke corresponding to toothto tooth spacing on the toothed faces of the parts such that eachpressure cycle will advance the ratch member one tooth length.
 23. Thearrangement of claim 22, wherein the toothed face and the piston of thefirst part are separable.
 24. A ratchet arrangement for downholeapparatus, the arrangement comprising first and second parts, each partdefining a toothed face, and a ratch member located between the toothedfaces, the parts being axially relatively movable, wherein reciprocalmovement of the parts advances the ratch member axially along thetoothed faces and wherein at least the toothed faces of the first andsecond parts and the ratch member are part of a unit which is separablefrom the tool with which the arrangement is associated.
 25. A ratchetarrangement for downhole apparatus, the arrangement comprising first andsecond parts, each part defining a toothed face, and a ratch memberlocated between the toothed faces, the parts being axially relativelymovable, wherein reciprocal movement of the parts advances the ratchmember axially along the toothed faces wherein the ratch membercomprises first and second portions and a spring portion actingtherebetween to urge the first and second portions into engagement withthe respective toothed faces.
 26. The arrangement of claim 25, whereinthe ratch member portions are combined as a single integral part.
 27. Adownhole valve comprising: a body defining a bore; a valve closuremember positioned in the bore; first and second retaining memberspositioned on respective sides of the valve closure member for retainingthe valve closure member in a closed position and to hold pressure fromboth sides, one of the retaining members being retractable to permitopening of the valve closure member; and means for locking said one ofthe retaining members in a retaining position, said locking means beingreleasable by application of a plurality of pressure cycles.
 28. Amethod of completing a downhole string including the steps of: providinga valve in a tubular string defining a bore, which valve is capable ofholding pressure from both above and below and is openable byapplication of a plurality of pressure cycles to the string bore;running the string into a bore with the valve closed; applying testpressures to the string bore above the closed valve; securing the stringin the bore; and applying one or more pressure cycles to the string boresubsequent to applying said test pressures to open the valve to permitflow of fluid through the string.